/**
 * 地图工具类
 */
var EARTH_RADIUS = 6378137.0;//地球半径，单位:米
var PI = Math.PI;
function getRad(d){
    return d * PI / 180.0;
}
/**
 * 根据经纬度求距离
 * @param lat1
 * @param lng1
 * @param lat2
 * @param lng2
 * @returns
 */
function getDistanceByLatLng(lat1,lng1,lat2,lng2){
    if (lat1 == lat2 && lng1 == lng2){
        return 0;
    }
    var f = getRad((lat1 + lat2) / 2);
    var g = getRad((lat1 - lat2) / 2);
    var l = getRad((lng1 - lng2) / 2);
    
    var sg = Math.sin(g);
    var sl = Math.sin(l);
    var sf = Math.sin(f);
    
    var s,c,w,r,d,h1,h2;
    var a = EARTH_RADIUS;
    var fl = 1 / 298.257;
    
    sg = sg * sg;
    sl = sl * sl;
    sf = sf * sf;
    
    s = sg * (1 - sl) + (1 - sf) * sl;
    c = (1 - sg) * (1 - sl) + sf * sl;
    
    w = Math.atan(Math.sqrt(s / c));
    r = Math.sqrt(s * c) / w;
    d = 2 * w * a;
    h1 = (3 * r - 1) / 2 / c;
    h2 = (3 * r + 1) / 2 / s;
    
    return d * (1 + fl * (h1 * sf * (1 - sg) - h2 * (1 - sf) * sg));
}